This application claims benefit of Japanese Patent Application No. 2004-002782 filed on Jan. 8, 2003, the contents of which are incorporated by the reference.
The present invention relates to liquid crystal display devices and, more particularly, to active matrix type liquid crystal display devices in lateral electric field system capable of obtaining high aperture factor and high contrast.
The TN (twisted nematic) system which has heretofore been broadly used, ensures high contrast. On the demerit side, however, the molecular axis of liquid crystal is raised by perpendicular electric field. This means a problem of pronounced visual field angle dependency. Recently, demands for systems for large size monitors such as TV sets are increasing, and an IPS (In-Plane Switching) system has been broadly adopted. In the IPS system, for the display the molecular axis of the liquid crystal is rotated by lateral electric field in a plane parallel to substrate. Thus, the visual field angle dependency on the rising angle of the molecular axis is precluded, and the visual field angle characteristic is greatly improved compared to the TN system.
While the IPS system liquid crystal display device features visual field angle characteristic improvement, it has the following problems. In this system, pixel electrode and common electrode for each pixel are arranged in a comb-teeth fashion. Therefore, the electrode area ratio to the display area is high, and the aperture factor is low. In addition, since the system adopts the lateral electric field drive, the liquid crystal molecules in the display area are adversely affected by the leaking electric field from the video signal line, thus giving rise to occurrence of longitudinal crosstalk.
Some solution measures with respect to the above problems have been proposed. One of such measures is disclosed in a prior art liquid crystal display device described before with reference to FIGS. 10A and 10B (see Literature 1: Japanese Laid-open 2002-323706, for instance). FIG. 10A is a plan view, and FIG. 10B is a sectional view showing a specified parts. A scanning signal line 1001 and a common signal line 1002 are formed from a first metal layer. A first insulating film 1003 is formed on the scanning signal line 1001 and the common signal line 1002. On the first insulating film 1003 a video signal line 1004, a thin film transistor 1005 and a source electrode 1006 are formed from a second metal layer. A second insulating film 1007 is formed on the video signal line 1004, the thin film transistor 1005 and the source electrode 1006. A transparent insulating film 1008 is then coated on the entire surface of the second insulating film 1007. A pixel electrode 1009 and a common electrode 1010 are then formed as transparent electrodes on the third insulating film 1008. The video signal line 1004 is perfectly covered by the common electrode line 1010 via the second and third insulating films 1007 and 1008. The pixel electrode 1009 and the common electrode 1010 are electrically connected via contact holes 1011 and 1012 to the source electrode 1006 and the common signal line 1002, respectively.
With the comb-teeth arrangement of the pixel electrode 1009 and the common electrode 1010 both formed as transparent electrodes, the areas on the electrodes contribute to the transmittivity. It is proved by simulation that, considering the contribution on the transparent electrodes, the effective aperture factor is improved by about 8%. With the arrangement that the video signal line is perfectly covered by the common electrode, it is possible to expand the aperture part up to the vicinity of the video signal line. In addition, with the shielding of the leakage electric field from the video signal line by the common electrode, an effect of reducing the longitudinal crosstalk is obtainable. This arrangement generates a load capacity between the video signal line and the common electrode. However, with the presence of the low dielectric factor insulating film, it is possible to suppress the load capacity down to a range which is free from any problem in the driving.
FIGS. 11A and 11B disclose a liquid crystal display device based on a different solution measure, which was proposed by the same Applicant as that of this application and was not published (not prior art), (Literature 2: Japanese Patent Application No. 2003-076169, for instance). FIG. 11A is a plan view, and FIG. 11B is a sectional view showing specific parts. A scanning signal line 1101 and a common signal line 1102 are formed from a first metal layer. A first insulating film 1103 is formed on the scanning signal line 1101 and the common signal line 1102. A video signal electrode 1104, a thin film transistor 1105 and the source 1106 are formed from a second metal layer on the first insulating film 1103. A second insulating film 1107 is formed on the video signal line 1104, the thin film transistor 1105 and the source electrode 1106. A third insulating film 1108 is coated on the second insulting film 1107. The third insulating film 1108 is formed in a bank-like fashion so that it remains on only the video signal line 1104. Then, a pixel electrode 1109 and a common electrode 1110 are formed as transparent electrodes. The video signal line 1104 is perfectly covered by the common electrode 1110 via the second and bank-like third insulating films 1107 and 1108. The pixel electrode 1109 and the common electrode 1110 are electrically connected via contact holes 1111 and 1112 to the source electrode 1106 and the common signal line 1102, respectively.
This solution measurement is different from that disclosed in Literature 1 in that the third insulting film 1108 is formed in a bank-like fashion so as to remain on only the video signal line 1104, while it is the same in such performance as the effective aperture factor. With this measure, since the third insulating film 1108 does not remain on the aperture part, it is possible to use a colored film as the third insulating film 1108. In the case of the full surface coating, the third insulating film 1108 has to be necessarily transparent. However, acrylic acid organic films and the like are expensive. On the other hand, novolak organic films or like colored films are inexpensive, and permit realizing the comparative performance at low cost.
This prior art technique, however, has other problems. With the arrangement in which the transparent insulating film is coated on the entire surface as disclosed in Literature 1, it is empirically proved that the extent of extension of the common electrode from each edge of the video signal line should be at least 6 μum in order to be able to sufficiently shield the leaking electric field from the video signal line. In the Literature 1 prior art technique, the electric field shielding effect is improved owing to the sectional shape of enclosing the video signal line. However, like the Literature 12 prior art technique, the pixel electrode and the common electrode are both formed as uppermost layers. This means that at least two contact holes are necessarily pixel. In either case, the narrower the pitch, the ratio of occupation of the extension of the common electrode and the contact holes in the ratio is the greater, leading to less effect of aperture factor improvement. Also, the narrower the pitch, the adverse effects of the leaking electric field from the video signal line are the greater, leading to less effect of the longitudinal crosstalk reduction.
As a measure for solving the above problems, a liquid crystal display device as shown in FIGS. 12A and 12B is disclosed, which was proposed by the same Applicant as that of this application and was not published (not prior art), (Literature 3: Japanese Patent Application No. 2002-164681, for instance). FIG. 12A is a plan view, and FIG. 12B is a sectional view showing specified parts. A scanning signal line 1201 and a common signal line 1202 are formed from a first metal layer on a first substrate. A first insulating film 1203 is formed on the scanning signal line 1201 and the common signal line 1202. On the first insulating film 1203 is formed, from a second metal layer, a pixel electrode 1206 which is integral with a video signal line 1204, a thin film transistor 1205 and a source electrode. With this arrangement, a pixel electrode 1206 requires no contact hole. A second insulating film 1207 is formed on the pixel electrode 1206 integral with the video signal line 1204, the thin film transistor 1205 and the source electrode. A third insulating film 1208 is coated on the entire surface of the second insulating film 1207. If a common electrode is formed as a transparent electrode on the third insulating film 1208, it greatly increases the drive voltage due to the fact that it is a comb-teeth line formed from a different layer. Accordingly, for the purpose of reducing the load capacity, the third insulating film 1208 is formed in a bank-like shape so that it remains only on the video signal line 1204. Then, the common electrode 1210 is formed as a transparent electrode. The common electrode 1210 is electrically connected via the contact hole 1212 to the common signal line 1202. Like the Literature 2 prior art technique, the third insulating film 1208 may be transparent or colored.
Like the Literature 2 prior art technique, the video signal line is perfectly covered by the common electrode via the second and bank-like third insulating films. Also, like the Literature 2 prior art techniques, the video signal line has a sectional shape enclosed by the common electrode. Thus, it is possible to obtain an improved effect of shielding the leaking electric field from the video signal line compared to the Literature 1 prior art technique. It is empirically proved that for sufficiently shielding the leaking electric field from the video signal line, the extension of the common electrode from each edge of the video signal line may be 4 μm.
A shown, the number of necessary contact holes is reduced by one, and also extension of the common electrode from each video signal line edge can be reduced. It is thus possible to obtain high aperture factor. However, since the pixel electrode is formed as a metal electrode, the contribution on the transparent electrodes is lower than in the prior art example. However, simulation proves that the effective aperture factor is increased by 5%. In consequence, it is possible to obtain improved aperture factor and more suppress longitudinal stroke compared to the Literatures 1 and 2 prior art techniques.
As shown above in connection with the Literature 1 to 3 prior art techniques, the third insulating film interposed between the video signal line and the common electrode covering the same, are formed in a bank-like fashion along the video signal line, it is possible to realize cost reduction and aperture factor improvement.
However, with the third insulating film left in the bank-like fashion only on the video signal line for improving the performance with aperture factor improvement, the step in this part is increased. Therefore, at the rubbing time the orientation becomes non-uniform in this part, and the initial orientation is disturbed. Such disturbance is observed as light leakage from the sides of the step part. In the IPS system which adopts normally black drive, the disturbance of the initial orientation causes light leakage in the vicinity of the step of the third insulating film at the black display time. Therefore, the black luminance is increased to lead to contrast reduction.